linux-kernel - Re: Re: linux-next: build warning after merge of the bpf-next tree

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Message-ID: <CAADnVQLo5HEjTpTTRm=BtExuKifPtCJm+Hu_WP6yeyV-Er55Qg@mail.gmail.com>
Date:   Fri, 10 Jan 2020 15:18:50 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Alexandre Ghiti <alexandre@...ti.fr>
Cc:     Stephen Rothwell <sfr@...b.auug.org.au>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Networking <netdev@...r.kernel.org>,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-arm-kernel@...ts.infradead.org, zong.li@...ive.com,
        Andrii Nakryiko <andrii.nakryiko@...il.com>
Subject: Re: Re: linux-next: build warning after merge of the bpf-next tree

On Fri, Jan 10, 2020 at 2:28 PM Alexandre Ghiti <alexandre@...ti.fr> wrote:
>
> Hi guys,
>
> On 10/27/19 8:02 PM, Stephen Rothwell wrote:
> > Hi all,
> >
> > On Fri, 18 Oct 2019 10:56:57 +1100 Stephen Rothwell <sfr@...b.auug.org.au> wrote:
> >> Hi all,
> >>
> >> After merging the bpf-next tree, today's linux-next build (powerpc
> >> ppc64_defconfig) produced this warning:
> >>
> >> WARNING: 2 bad relocations
> >> c000000001998a48 R_PPC64_ADDR64    _binary__btf_vmlinux_bin_start
> >> c000000001998a50 R_PPC64_ADDR64    _binary__btf_vmlinux_bin_end
> >>
> >> Introduced by commit
> >>
> >>    8580ac9404f6 ("bpf: Process in-kernel BTF")
> > This warning now appears in the net-next tree build.
> >
> >
> I bump that thread up because Zong also noticed that 2 new relocations for
> those symbols appeared in my riscv relocatable kernel branch following
> that commit.
>
> I also noticed 2 new relocations R_AARCH64_ABS64 appearing in arm64 kernel.
>
> Those 2 weak undefined symbols have existed since commit
> 341dfcf8d78e ("btf: expose BTF info through sysfs") but this is the fact
> to declare those symbols into btf.c that produced those relocations.
>
> I'm not sure what this all means, but this is not something I expected
> for riscv for
> a kernel linked with -shared/-fpie. Maybe should we just leave them to
> zero ?
>
> I think that deserves a deeper look if someone understands all this
> better than I do.

Are you saying there is a warning for arm64 as well?
Can ppc folks explain the above warning?
What does it mean "2 bad relocations"?
The code is doing:
extern char __weak _binary__btf_vmlinux_bin_start[];
extern char __weak _binary__btf_vmlinux_bin_end[];
Since they are weak they should be zero when not defined.
What's the issue?